Is Fructose the Reason You're Not Losing Weight?

Fructose freak-out! New research suggests fructose—a type of sugar found in fruit and other foods—may be particularly bad for your health and waistline. But don't blame blueberries or oranges for your weight issues just yet.

First, the research: Researchers from the University of Illinois at Urbana-Champaign fed mice a diet in which 18 percent of the calories came from fructose. (This percentage is roughly the amount found in the average American kid's diet.)

Compared to mice whose diet included 18 percent glucose, another type of simple sugar found in food, the mice who ate fructose gained more weight, were less active, and had more body and liver fat after 10 weeks. This was despite the fact that all the mice in the study ate the same number of calories, the only difference was which type of sugar they consumed.(Here's A Sweet Reason to Sweat—cardio and resistance training can help negate the effects of sugar.)

So, basically, this research suggests that fructose could cause weight gain and health problems even if you're not overeating. (Yes, this was an animal study. But the researchers used mice because their little bodies break down food a lot like our human bodies do.)

That could be concerning, because you'll find the sweet stuff in many fruits, some root vegetables, and other natural foods. It's also a major component of artificial sweeteners, including table sugar and high-fructose corn syrup (which you'll find in everything from bread to barbecue sauce), says Manabu Nakamura, Ph.D., an associate professor of nutrition at the University of Illinois at Urbana-Champaign.

While Nakamura wasn't involved with this latest mouse study, he's conducted a ton of research on both fructose and other simple carbohydrates. "Fructose is primarily metabolized by the liver, whereas the other sugar, glucose, can be used by any organ in our body," he explains.

Here's why that's bad: When you consume a large amount of fructose, your overwhelmed liver breaks it down into glucose and fat, Nakamura says. Not only could this lead to weight gain, but that breakdown process can also mess with your blood's insulin and triglyceride levels in ways that may raise your risk for diabetes or heart disease, he explains.

Fortunately, the fructose in fruit isn't a problem. "There is no health concern at all about the fructose in whole fruits," Nakamura says. Not only is the amount of fructose in produce fairly low, but the fiber in many types of fruit also slows down your body's digestion of the sugar, which spares your liver a big rush of the sweet stuff. The same is true of fructose in root vegetables and most other natural food sources.

Swallowing treats or drinks packed with table sugar or high fructose corn syrup, however, could be a problem. These contain highly-concentrated doses of fructose, which flood your liver in a rush, says Nyree Dardarian, R.D., director of the Center for Integrated Nutrition & Performance at Drexel University. "Soda is the largest contributor to fructose consumption," she says.

Fruit juice also packs a pretty stout portion of both fructose and calories, and doesn't provide the digestion-slowing fiber of whole fruits, Dardarian says. But unlike soft drinks, you get a lot of healthy vitamins and nutrients from 100 percent fruit juice.

While she recommends completely cutting all sugary drinks from your diet, Dardarian advises keeping your juice habit to eight ounces of 100 percent pure fruit juice a day. (Why 100 percent pure? A lot of drinks contain a little fruit juice, supplemented with sugar or high-fructose corn syrup. Those are about as bad for you as soda.)

Bottom line: Big, concentrated doses of fructose appear to be bad news for your health and waistline. But if you're eating healthy fructose sources like fruit or vegetables, you have nothing to fear, Dardarian says. (If you're really worried about your sugar intake, try A Taste of a Low-Sugar Diet for a trial run.)